Dispersions especially concentrated dispersions are of obvious wide industrial importance in diverse applications such as foods, coatings, fabricated materials and consumer packaged goods. It is well known that the overall characteristics exhibited by such dispersions are in varying degrees controlled by the basic physical properties of the individual particles comprising the dispersion. These properties include the surface chemistry, size, surface area, volume fraction and shape of the particles making up the dispersed phase. Such basic physical properties are generally optimized for a specific application empirically by manipulation of process and composition until the desired characteristics of the dispersion as a whole are achieved, e.g., stability, viscosity, spreadability.
The optimization process itself and the control of the quality of the optimized dispersions, e.g., paint composition, can be greatly facilitated by being able to measure the basic physical properties of the dispersed and continuous phase(s) in an accurate and reproducible way. Indeed, a host of methods and specialized instruments have been introduced over the last 50 years each having their own range of applications, limitations, and assumptions. For example, these include instruments based on dynamic and static light scattering, electrical properties, and acoustics.
Low-field pulsed NMR has been recognized as a potentially powerful technique for the characterization of dispersions since the pioneering work of Tanner and Stejskal (J. Phys, Vol 42, P288 (1965)) and is now widely used to measure for example the size of water droplets in margarine and food spread compositions. Numerous modifications and improvements in the analysis of the relationship between NMR spectra and the particle or other basic physical properties of a dispersion have appeared in the literature in order to make the technique more accurate and reproducible.
Pulsed NMR is an attractive technique for the characterization of dispersions for a number of reasons. Firstly, it is capable of measuring a number of key physical properties as will be discussed further below. This allows in principle one instrument to perform the function of several more specialized instruments thus streamlining the characterization process and potentially the expense of setting up a characterization lab.
Secondly, pulsed NMR allows the characterization of dispersions without the need for dilution, or special sample preparation. For example, the technique can in principle be applied to dispersions containing solid, liquid or gas dispersed phases, concentrated dispersions, optically opaque dispersions, highly viscous dispersions and dispersions containing multiple dispersed phases.
In spite of its potential advantages, pulsed NMR has not gained broad acceptance as a routine technique for the characterization of broad classes of dispersions. Its main use has been in the characterization of droplet size, water content of water-in-oil emulsions, and oil characterization, e.g., solid/liquid ratio, especially in the food and petroleum industry.
Among the potential reasons for the limited use of NMR in particle characterization is the lack of availability of compact, portable and readily affordable instruments that are specifically designed for dispersion characterization. Currently available instruments tend to be bulky, heavy and expensive.
Furthermore, currently available low field NMR instruments are primarily designed as general purpose instruments. As such these instruments are not optimized for dispersion characterization. Since the science underpinning pulsed NMR is complex, especially the relationships between pulse sequencing, signal acquisition and dispersion characteristics, the task of adapting these instruments for routine analysis is often daunting.
The following patents and publications form a part of the related art:
U.S. Pat. No. 4,389,613 and U.S. Pat. No. 4,480,227 to Brown describe portable pulsed NMR instruments and method of use that are designed to measure fluid flow properties in porous media especially oil field rock samples.
Shanks et al in a publication entitled “Miniature magnet assembly for NMR-ESR spectroscopy” (Am. J. Phys, Volume 48, pp 620-622) describes the evaluation in terms of field strength and field uniformity of the magnet assembly incorporating a sintered SmCo3 pole pieces. The assembly has a field strength of 3.35 kG with a claimed uniformity of +/−13.4 mG.
Martin et al. in a publication entitled “The NMR mouse: Its application to food science” (Magnetic Resonance in Foods Science: Latest developments, edited P S Belton, Royal Society of Chemistry, 2003) describes a low field bench top pulsed instrument based on a one-sided magnet and surface mounted RF coils. The magnet arrangement provides a high but rapidly decaying magnet field adjacent to one of the poles of the magnet.
Packer and Rees report the use of a pulsed magnetic field-gradient spin echo technique for the determination of droplet size distribution in emulsions (J. Colloid and Interface Sci., vol. 40, p206 (1972)).
Van Den Enden et al. describe a protocol for the rapid determination of water droplet size distributions in water-in-oil emulsions by pulsed field gradient NMR utilizing echo attenuation caused by restricted diffusion (J. Colloid and Interface Sci., vol. 140, p105 (1990)).
Goudappel et al. developed a method for measuring oil droplet size in oil-in-water by pulsed field gradient NMR by suppression of the NMR signal from the continuous water phase (J. Colloid and Interface Sci., vol. 239, p 535 (2001)). The accuracy of this technique was subsequently checked by Denkova et al. (Langmuir, vol. 20, p11402 (2004).